Current directional microphones for hearing aids are made by matching a pair of microphones on phase and amplitude with a certain accuracy and specification. The main problem is drift after the matching. One of the major reasons for drift is changes in membrane tension. Both membranes will never age in the same way.
The technical problems of the state of the art are to control long and short term drift of the microphones. For short term drift due to say humidity, temperature etc. materials may be selected which have low drift coefficients. For longer term drift, other materials may be required, as any plastic material will have aging problems.
One prior art solution, naturally is the pairing of neighbouring membranes in a production matrix. This, however, has been found insufficient.